ALBERT

Description: The feeding ecology of Greenland halibut (Reinhardtius hippoglossoides) on the deep slope of the northeast Newfoundland continental shelf was investigated based on stomach analysis. Stomach samples were collected from the spring 1992 Canadian bottom trawl fishery at depths of 1000–1250 m. The squid, Gonatus spp., predominated in the diet, representing the first known instance of reliance of Greenland halibut upon this bathypelagic squid as prey in Canadian waters. A change from predation primarily upon pelagic prey, reported in earlier studies, may be related to annual variability in abundance and distribution of both Greenland halibut and its principal pelagic fish prey species, capelin (Mallotus villosus). Seasonal effects may account for the absence of epipelagic, juvenile Gonatus spp. from the diet. There was a linear size relationship in observed predation which may reflect a common size-related depth distribution pattern between predator and prey or, more likely, prey selectivity. At larger sizes, Greenland halibut switched from Gonatus spp. to groundfish as its principal prey. The importance of Gonatus spp. in the deep continental slope trophic web is reviewed.

Description: Pico- and ultraplankton are known to contribute significantly to overall biomass and primary productivity in the 'high nutrient low chlorophyll' waters of the equatorial Pacific. In order to understand the dynamics of this community on ecologically relevant time-scales, we examined the abundance, distribution and cellular characteristics of Prochlorococcus, Synechococcus, eukaryotic ultraphytoplankton and heterotrophic bacteria during two 20-day time-series at 0°N, 140°W in the spring and fall of 1992 (JGOFS time-series cruises. TS-I and TS-II). Prochlorococcus was numerically dominant among the autotrophic groups considered, with mean cell concentrations in surface waters on the order of 1.4 x 105 cells ml-1. Synechococcus and ultraphytoplankton abundances were 17-30-fold lower than those of Prochlorococcus, and heterotrophic bacterial abundances were 5-7-fold higher (during TS-I and TS-II, respectively). Daily cell abundances for all groups varied by factors of 1.5-2 within each time-series. Depth-integrated Prochlorococcus abundance averaged over each time-series was 25 lower during TS-II relative to TS-I; ultraphytoplankton abundance was 42 higher during the same period. Prochlorococcus and ultraphytoplankton both contributed significantly to the estimated total autotrophic biomass; Synechococcus contributed relatively little. Estimated total photosynthetic pico- plus ultraplankton biomass was on average 30 higher than heterotrophic bacterial biomass. Changes in the fluorescence and light scatter properties of individual Prochlorococcus cells were observed during the passage of a tropical instability wave during TS-II, and are hypothesized to reflect a physiological response among these cells to that event Examination of bulk properties alone (e.g. cell numbers or total red fluorescence) would not have revealed these physiological changes. Lower bounds for Prochlorococcus-specific growth rates were calculated based on the DNA distributions of these populations at dusk. These rates were maximal at 15 or 30 m depth, where they approached one doubling per day. Changes in Prochlorococcus forward tingle light scatter (FALS) from dawn to dusk were well correlated with these estimates of specific growth rate, an observation that allowed us to relate measurements of FALS to cell volume for Prochlorococcus.